Stark tuning rate

Figure 6. Infrared COl stretching frequency and Stark tuning rate as a function of particle size, determined from SNFTIR spectra of CO species resulting from the adsorption of methanol from 0.5 M CH3OH in 0.1 M HCIO4 solution. ...

Potential-dependent frequencies in spectra of adsorbates in electrochemical interfaces are commonly observed. Thus so-called Stark tuning rates, Qv/QE, of 30cm have been reported for adsorbed CO on platinum [65, 111] (Fig.59) and adsorbed CN on silver [109, 111, 162]. Even higher values were found for sulfate species adsorbed on polycrystalline platinum (100 cm V [36, 38]) or on single crystal Pt(lll) [141, 143] (120cm V ). In some cases, as for adsorbed tetra-cyanoethylene [163] and anthracene [164], vibrational features which are forbidden by the surface selection rule become active under the influence of the electric field at the interface. 

To a good approximation the Stark tuning rate is given by... 

Applying the model to adsorbed CO, Lambert has evaluated the coefficients a from experimental data. Thus 020 was taken from the unperturbed frequency (vq), 03 0 from gas-phase values of a Morse potential, o, j from the IR cross-section of adsorbed CO and 021 from the first overtone observed by EELS. Equation (18) has been applied successfully to CO/Ni(lll) and CO/Pt, both in the gas phase and at electrodes, reproducing the observed Stark tuning rate with a good accuracy. 

It is very interesting that the Stark tuning rate is coverage-dependent for adsorbed CO, in electrochemical media [55-57] as well as in vacuum [172]. It is found that at lower degrees of coverage the Stark tuning rate increases significantly. This depen-... 

Under UHV, the proportionality factor between the vibrational frequencies and the local electric field is termed the Stark-Tuning Rate (STR), 6ve, and has units cm . (V.cm" ) . Note that this factor is also the one which is detamined by quantum chemical calculations. In solution, the accessible quantity is the Stark-Tuning Slope (STS), dvf, expressed in cm. V . We shall extract the STR by multiplying this latter quantity by the thickness k of the double layer [34] ... 

Table 1 Experimental Stark-Tuning Slopes (in cnr ,V ) for CO adsorbed on various metallic surfaces dipped into various solvents. The theoretical values are those obtained in this work, using the presently calculated Stark-Tuning Rate along with a thickness d (in A) for the EDL calculated via the Gouy-Chapmann model.

Stark-Tuning Rates and Stark-Tuning Slopes for CO on Pd(lOO)... 

Tables Computed Stark Tuning Rates (in cm. (V.cm y O for different Pd CO clusters, for both the Pd-C and C-0 stretching modes.

Because the vibrations that underlie IR absorption spectra must affect the electric dipole of a molecule, we would expect the frequencies of these modes to be sensitive to local electric fields, and this is indeed the case. Shifts in vibration frequencies caused by external electric fields can be measured in essentially the same manner as electronic Stark shifts, by recording oscillations of the IR transmission in the presence of oscillating fields. The Stark tuning rate is defined as S = dv/dE , where v is the wavenumber of the mode and is the projection of the field (E) on the normal coordinate [88, 89]. To a first approximation, is given by —u Aft + E Aa)/hc, where ti is a unit vector parallel to the normal coordinate, Afi is the difference between the molecule s dipole moments in the excited and groxmd states, and Aa is the difference between the polarizability tensors in the two states (Sect. 4.13, Box 4.15 and Box 12.1). However, anharmonicity and geometrical distortions caused by the field also can contribute to vibrational Stark effects [90, 91]. 

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